Decatrienes twist asynchronicity

Similar trends, albeit less pronounced in several instances, are apparent in the data summarized in Table 2 for ( , , )-decatrienes. Houk has attributed the smaller effect of the EtaN substituent in the deca- compared to the nona-triene series to the smaller requirement for twist asynchronicity in the deca-triene transition state. It is argued that this transition state is essentially unstrained by the four-carbon connecting chain and that less torque is applied to the developing C(2)—C(7) bond. - Although the... 

Increased endo stabilization of the c(.r-fused transition state must be invoked to account for the increased selectivity in the Lewis acid catalyzed cyclization of the (Z, , )-decatrienoate recorded in Table 2, entry 8. Because twist asynchrrmicity is believed to be less important in the decatriene transition state, Alder endo stabilization apparently dominates twist asynchronicity in this case. 

The increased stereoselectivity of decatrien-3-ones relative to the n(matrien-3-ones may also be the consequence of cooperativity between twist asynchronicity and endo stabilization that both favor the c -fused product. The decatrien-3-one transitirm state is considerably less strained than the nonatrien-3-one transition state, and the uncatalyzed decatrienone cyclizations occur at or near ambient temperature. Consequently, one expects greater endo stabilization, and hence also greater cis stereoselectivity, in the decatrienone intramolecular Diels-Alder reactions. 

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